I think Kalze is looking for a guide to show where you have to shoot the enemies to get a headshot (probably some pictures showing which zones will count as a headshot).
Toumaz is saying that if you get a headshot, your bullet trail turns slightly yellow. I went to check this out and.. it's true! Although the bullets move so fast, and the yellow is so faint, I could only really see it when I got a headshot from across the screen. I like the idea though, perhaps in the next release the shade of yellow could be a little darker?